The process in which oxidation number increase, is

Options

A. Reduction
B. Hydrolysis
C. Oxidation
D. Decomposition

Answer

To solve the question "The process in which oxidation number increases is:", we will analyze the options provided and determine which one correctly describes the process of an increase in oxidation number.

### Step-by-Step Solution:

1. **Understanding Oxidation and Reduction**:
- Oxidation refers to the loss of electrons by a substance, which results in an increase in its oxidation number.
- Reduction refers to the gain of electrons by a substance, which results in a decrease in its oxidation number.

2. **Evaluating the Options**:
- **Option A: Reduction**:
- As discussed, reduction involves a gain of electrons, leading to a decrease in oxidation number. Therefore, this option is incorrect.

- **Option B: Hydrolysis**:
- Hydrolysis is a chemical reaction involving water that breaks down a compound. It does not specifically relate to changes in oxidation numbers, so this option is also incorrect.

- **Option C: Oxidation**:
- Oxidation involves the loss of electrons, which results in an increase in oxidation number. For example, if copper (Cu) is oxidized from Cu⁺ to Cu²⁺, its oxidation number increases from +1 to +2. This option is correct.

- **Option D: Decomposition**:
- Decomposition refers to the breakdown of a compound into simpler substances. It does not specifically relate to changes in oxidation numbers, so this option is incorrect.

3. **Conclusion**:
- The process in which the oxidation number increases is correctly identified as **oxidation**.

### Final Answer:
The process in which oxidation number increases is **oxidation**.